The scale on a map is 5cm : 9km. If the distance between two cities is 81km, how far apart in cm are the two cities on the map?
The scale means that every 5cm on the map corresponds to an actual distance of 9km. An actual distance of 81km is the same as 9 $\cdot$ 9km. The distance between the two cities on the map is 9 $\cdot$ 5cm, or 45cm.